Credit card or prepaid card: Problem with a purchase shown on your statement

In XX/XX/2019 I opened a secured credit card account with CitiBank in the amount of {$1000.00}. Under the agreement of the secured account my credit limit would be equal to my deposit. I have paid my monthly bill on time every month without any problems. On XX/XX/2019 Citibank Approved a transaction in the amount of {$2200.00} without my approval and a amount that was {$1200.00} over my credit limit. The charge was a overcharge from a car rental place. I filed a dispute and Citibank sided with the car rental place, even after admitting I never gave the car company authorization. Citibank has the paperwork that clearly shows I never signed the agreement. Citibank will not fix the problem or explain how My account was charged {$2200.00} with my credit limit only being {$1000.00}. Whose to stop them from approving a {$100000.00} charge then telling me I owe them? This charge should had never been approved and definitely should not have sides with the merchant if the agreement was never signed. I have the documents but Citibank uses a complicated process for documents that prevents you from downloading or printing the files?